Mean drop volume emulsions

Industrial liquid-liquid extraction most often involves processing two immiscible or partially miscible liquids in the form of a dispersion of droplets of one liquid (the dispersed phase) suspended in the other liquid (the continuous phase). The dispersion will exhibit a distribution of drop diameters d, often characterized by the volume to surface area average diameter or Sauter mean drop diameter. The term emulsion generally refers to a liquid-liquid dispersion with a dispersed-phase mean drop diameter on the order of 1 pm or less. 

Let two concentrated dispersions with the same type of continuous phase (e.g., an aqueous foam and an 0/W emulsion, or two different OAV emulsions) be brought into contact, either directly or via a freely movable semipermeable membrane. If the osmotic pressures are unequal (e.g., as a result of differences in the volume fractions, mean drop size, interfacial tension, or combinations thereof), it is obvious that the (common) continuous phase will flow from the dispersion with the lower osmotic pressure into that with the higher osmotic pressure until flie two pressures are equalized. The final volumes and volume fractions of the two dispersions may be predicted in a straightforward manner, once n( ) known. It is important to point out that equality of the (mean) capillary pressures does not necessarily rule out flow, nor does their inequality imply it. 

For 0 > 0.80, the results of the two studies appear to be quite consistent, in spite of the disparity in the degree of polydispersity of the emulsions employed. The apparent discrepancy at the lower volume fractions may be entirely the result of the large differenee in mean drop size, for the reasons cited above. 

Figure 3 Experimental data for creaming in xylene-in-water emulsions. The volume of the transparent semm left below the creaming emulsion, scaled with the total volume of the liquid mixture, is plotted against the time elapsed after ceasing the agitation. The emulsion is stabilized with 3-lactoglo-bulin, whose concentrations, corresponding to the separate curves, are shown in the figure. The empty and full symbols denote, respectively, coarse emulsion (mean drop size 5 pm) and fine emulsion (mean drop size 0.35 pm).

Phase separation (route 1 in Figure 8.2) means that an emulsion may separate into two phases one phase enriched with the droplets and the other enriched with the continuous phase. These kinds of separation are called creaming or sedimentation if the upper phase and the lower phase is formed by the drops, respectively. Note that the sediment or the cream are themselves both dispersions, but with a much higher volume fraction of the dispersed phase. The phase separation is macroscopic and does not lead to a coalescence of the dispersed drops. The stationary velocity for laminar conditions during phase separation... 

Let us assume that the particles are randomly distributed across drops irrespective of drop volume, so that even in antifoam emulsion the volume size distribution of drops is the same irrespective of the presence or absence of particles. The probability of obtaining particle-free drops is then equal to the total volume fraction of such drops (so that go = o)- I which case, we calculate from Equation 6.4 that N/M 0.46 if volume fractions of particle-free drops are to match the values of 0.63 indicated by experimental observation of the silica content of the agglomerates in deactivated antifoam dispersions. Since N/M < 1, this means that such high volume fractions are predicted to occur only in the case of dilute systems where the number of drops exceeds the number of particles. Obviously, this situation could be achieved as antifoam is dispersed where the number of drops increases, as a result of splitting, because the number of particles then remains constant so the ratio MM must decline. Here we note that if both particles and drops are spherical and monodisperse and the overall volume fraction of the particles is small, then... 

If designed and operated at design pressure drop, these static mixers can produce a narrow drop size distribution. For example, 70% of dispersed volume can be within 20% of the mean drop diameter. However, these mixers work poorly at low velocities, because the pressure drop can fall below the necessary level. Also, at extremely high velocities, there is a danger of stable emulsion formation. 

Figure 16 shows the results when 20 pore volumes of an emulsion having a 3.1- xm mean droplet size is injected into an 1170-mD sand pack and is followed by several pore volumes of water (ii). After emulsion injection, a permeability reduction of about 50% is observed. With water injection, the effluent concentration drops to 0 after one pore volume, whereas the permeability is unaltered. For this dilute emulsion, the droplets are captured in the porous medium, and this capture leads to blocking of the flow paths. Figure 16 shows that once the droplets are captured, they do not re-enter the flow stream, velocity being constant. Soo and Radke (ii) proposed the following physical interpretation for the results of Figure 15. Initially oil droplets are preferentially captured in the small-size pores, and as injection proceeds, more and more of the small pores become blocked. This blockage leads to a flow diversion toward larger size pores, and the rate...

Where is the phase volume ratio of the 0/W emulsion and n is the number of drops for mean volume, Vm, at time, t, is the flocculation rate constant. 

McClements and Dungan reported, based on light scattering measurements, that the Sauter or surface-volume mean diameter of drops in a dilute emulsion of n-hexadecane in water remained constant while the number of drops decreased with time during solubilization of the hydrocarbon into a 2 wt% solution of Tween 20 (sorbitan monolaurate). Weiss et al. found similar results for the same surfactant with n-tetradecane and n-octadecane. This result, which seems surprising in... 

One of the unique rheological features of emulsions is that the apparent viscosity of the emulsion can drop below the viscosity of the continuous phase when the concentration of the dispersed phase is low, normally below 0.1 in volume fraction (194). When solids are added to the emulsion, the apparent viscosity can decrease even further and the volume fraction of the dispersed phase at which minimum viscosity occurs increases with increasing solids content. Figure 30 shows the apparent viscosity of water-and-sand-in-bitumen, pwsh, variation with the solid-free water volume fraction, j8w, for two shear rate values. The experimental data were provided by Yan (private communication), where the system consists of 52 pm sand particles treated with hexadecyltri-methylammonium bromide (HAB) and water droplets of a Sauter mean diameter of 9 pm dispersed in bitumen at 60 °C. The sand particle volume fraction on water-free basis is j8s = 0.193. The range of the water droplet volume fraction, on a solid-free basis, jfrw is between 0 and 0.4. It can be observed that a minimum viscosity is present at a solid-free water droplet volume fraction of about 0.1. For a lower solid concentration, Ps = 0.113, the minimum apparent viscosity is found at /3W = 0.05... 

Volume percent of inner aqueous phase in Wi/O emulsion Volume percent of Wi/O emulsion drops in W1/O/W2 emulsion Mean size of inner aqueous phase Mean size of homogenized oil droplets... 

The results are reported of a systematic study of the break up of droplets in concentrated emulsions at different viscosity ratios in simple shear flows. The system investigated consisted of silicon oil drops in an aqueons phase mixture of polyacrylic acid solution, hexylene glycol, distilled water and dobanol surfactant. The ratio between drop and matrix viscosity was varied from 0.1 to 22 and the volume fraction ranged from 0 to 70%. The results are discussed in terms of a simple mean field scaling model. 11 refs. 

---